Hi Ceppad,
I also ran into this doubt sometime back. I did some research about what people think and all and I think the following links might help you,
1.
http://lists.oasis-open.org/archives/xacml/200107/msg00068.html 2.
http://csrc.nist.gov/rbac/sandhu96.pdf Actually, you will find that 1st link points to 2nd at the end
In the 2nd link's pdf look at section-3 where they try to specifically address the question you asked...
HTH
Maulin